The present invention relates to dual chamber pacemakers and, more particularly, dual chamber pacemakers which maximize the recognition of early atrial senses so as to detect tachycardia.
Tachycardia detection has assumed a role of high importance in design of modern dual chamber pacemakers. The ability to detect tachycardia episodes, collect data concerning such episodes which can be outputted to the physician, and to provide remedial responses where appropriate, has been the object of considerable design effort in the pacemaker area. The approach taken by Vitatron Medical, B. V., the assignee of this invention, has involved minimizing the atrial no-sense period (ANSP), so that most spontaneous early atrial events can be sensed. Thus, if theoretically no blanking period were ever used following an atrial sense, the pacemaker would be capable of collecting the maximum amount of information concerning early atrial senses, and thus would be in the best possible position to determine the onset of atrial tachycardia. Of course, such a pacemaker would have to have sophisticated logic to determine when and how such early atrial senses can be used.
Our investigations have shown that such a pacemaker with an advanced arrhythmia detection capability can react poorly to stable first degree AV block ("AVB1"), under circumstances where an early atrial sense, such as a premature atrial contraction (PAC), is followed by a lengthened spontaneous AV conduction duration. In this situation, the pacemaker may be "trapped" in a situation initiated by the lengthened AV conduction duration where a next following atrial signal is sensed but arrives too early to permit the pacemaker to follow with a tracked ventricular pulse. Such an early atrial sense cannot be tracked because the ventricular pace pulse would be delivered at too short an interval compared with the prior ventricular event, i.e., it would correspond to an excessive ventricular rate. In a pacemaker such as described in U.S. Pat. No. 5,247,930, assigned to the assignee of this invention and incorporated herein by reference, there is established a dynamic tracking limit (DTL), and a condition for tracking an atrial sense is that the resulting ventricular rate cannot exceed DTL. Further, the AV interval cannot routinely be extended past a predetermined maximum, in order to prevent pacemaker mediated tachycardia (PMT). Thus, the combination of sensing high rate early atrial spontaneous beats, together with the built-in logic of controlling when the atrial event can and cannot be sensed, can lead to a problem when applied to patients with first degree AV block, namely that the pacemaker is "trapped" or "stuck" in a situation where atrial paces are not delivered since the spontaneous P-wave is seen, and ventricular paces are not delivered because this would result in an excessive ventricular rate. The result is that a single PAC, or incidental undersensing such as sometimes occurs in a VDD, can initiate a prolonged period of time during which the patient is suffering first degree block and the pacemaker does not intervene.
AVB1 can also occur in a conventional pacemaker which responds to a PVC by extending PVARP. In such pacemaker, after a PAC which occurs in the PVARP, and is not tracked, there can be a VS before timeout of the escape interval, which is seen as a PVC. When PVARP has been extended, the subsequent AS which reoccurs at a constant rate is blocked, resulting in another VS after a lengthened AV-delay, and so on. In this case also, it is seen that there is good reason to minimize the atrial blanking period so as to detect AVB1, and react quickly and actively. See also the paper titled "Apparent P-Wave Undersensing In A DDD Pacemaker Post-Exercise," van Gelder et al., PACE, Vol. 15, November 1992, Part I, pp. 1651-1656.
The above-noted problem which can arise in a patient with first degree AV block, and who has a pacemaker with a minimal ANSP, is illustrated in the timing diagram of FIG. 1A. In FIG. 1A, as well as the timing diagrams of FIG. 1B and FIG. 4, the following symbols and abbreviations are used:
______________________________________ ABBREV SYMBOL DESCRIPTION IATION ______________________________________ Normal Atrial Sense NAS Normal Ventricular Sense; a VS NVS with a V-V interval within the rate limits, which is preceded by an A.sub.-- Event. Premature Atrial Contraction